Conductors that couple a supply voltage to a load have a characteristic resistance. Current carried in a conductor from the supply voltage to the load, causes a voltage drop determined according to Ohm's Law (V=IR). The voltage level at a load coupled to a supply voltage through a conductor, is equal to the supply voltage level at some reference point, less a voltage drop equal to the product of the resistance of the conductor, times the current passing through the conductor.
Integrated circuit packages typically have power supply input pins (or contact pads, etc.) for coupling with one or more supply voltages and a ground potential so as to drive various internal loads. The operational loads to be powered from the power supply inputs to the integrated circuit package are coupled to the power supply inputs through conductors that trace across the area of the package and up or down through connections made between superimposed solid state semiconductor layers. The conductors may comprise narrow strips of thinly deposited metal alloys or other materials. Although the integrated circuit package as a whole is a load on the power supply, the operational devices within the package likewise are internal loads coupled through conductors to the power supply. The internal loads include local assemblies of cooperatively functioning circuit elements, namely semiconductors (transistors, diodes, etc.), resistors, capacitors and inductors. The functioning of the assemblies, for example as switching devices, amplifiers, comparators and similar elements, is affected by the power supply levels that are available locally within the integrated circuit package.
The materials of conductors that couple between the integrated circuit package power supply pins and the terminals of the functioning circuit elements have a characteristic resistivity. Due to material resistivity and conductor dimensions (length and lateral cross sectional area), there may be a considerable electrical resistance between the circuit package power supply pins and the terminals of the functional circuit elements, such as the collectors of transistors, the anodes and cathodes of diodes and similar local elements.
Assuming that the power supply input pins to the IC package are held at a nominal supply voltage and at substantial ground potential, respectively, internal elements will be subjected to a lower local supply voltage level and a higher local ground potential. As a result of the electrical resistance of the conductors that couple the supply voltage and ground potential to internal functioning circuit load elements, the local supply voltage at the internal functioning circuit elements is reduced, and the effective ground potential is increased, by an amount equal to the product of the current passing through the circuit element and the resistances of the conductors. The actual voltage difference applied across the circuit elements, which is the effective voltage with respect to circuit operation, is equal to the voltage difference across the input pins only when no current is running through the load. The local voltage drop across the loads varies dynamically in inverse relation to current.
According to operation of the integrated circuit, and in particular according to variations that occur over time, such as to the number and distribution of switching devices that are conducting or not conducting, the effective voltage differences available to the functioning circuit elements and subassemblies varies. According to Ohm's Law, the local supply voltage drop versus the nominal supply voltage (or the supply voltage “droop”) and the increase in local ground potential (ground “bounce”) varies with current. In an integrated circuit package, there may be numerous circuit assemblies or elements functioning variably as loads. A given circuit assembly or load may be more or less proximal to the integrated circuit package supply voltage inputs (which supply voltage inputs should be construed to encompass both the supply voltage input and the ground potential input), resulting in differences in series resistance. In addition to these factors, a particular load element or assembly may be closely adjacent along the voltage supply and ground conductors, to a different load element or assembly that switches in the extent of its current loading, so that the voltage difference across the particular load is dragged down due to current loading on the adjacent load or assembly.
The combined effects of these factors on a given load include a reduced voltage difference applied across the positive and negative terminals of local loads with increased current loading of that load and/or other nearby loads. The resistance of the conductors between the external supply voltage and the internal load elements result in changes in internal local supply voltages with changes in current, even assuming that the external supply voltage remains constant. The precise extent of the voltage supply droop and ground bounce is variable and can be complex, because they are determined in part by the operational states of the local assemblies within the integrated circuit package.
Circuits in general are designed with the assumption that they will be driven at a nominal potential difference, namely a given voltage between the points at which the active circuit elements are functionally coupled to their positive and negative supply voltage levels. Departure from a nominal potential difference may affect circuit elements in various ways according to their structure and function. For example, switching times may vary as a function of the effect of voltage differences on charging rates. Differences in switching voltage thresholds may arise when a supply voltage level shifts. Other things being equal, such variations in a digital circuit provide limits on the switching speeds and voltage levels at which the device can be operated dependably. For example, signal race anomalies may arise if different local elements vary in operational speed due to local departures from nominal supply voltage conditions. Oscillating circuit frequencies and one-shot time delays can vary. These factors can interfere with nominally correct operation of the integrated circuit device or its subassemblies.
The electrical resistance of a conductor such as a supply voltage conductor is a function of material resistivity, cross sectional area (width and thickness) and conductor length. In high speed digital integrated circuits such as processors, numerous densely placed switching transistors, drivers and related circuits, typically driven from one or two supply voltages and a ground voltage coupled to the integrated circuit through printed circuit board lands or other conductors, to a voltage supply. The conductors are thin deposited films and strips. The circuit elements differ widely in current loading levels, particularly with changing operational states of the device.
Typically, the supply voltage (often termed VCC and/or VDD) and the ground potential (GND) for an integrated circuit or subset of circuit elements therein, comprises one or more supply conductors forming a buss arranged to couple voltage and current commonly to a number of circuit elements. Such a buss is sometimes termed a “power rail” and may be configured as (or can be envisioned as) a ladder arrangement wherein the local load elements are ladder rungs and the positive and negative supply conductors are ladder stiles. The busses may or may not be laid out in a geometric arrangement as described, but in some digital circuits there are pronounced geometrical load placement patterns having attributes similar to a ladder arrangement.
Decoupling capacitors can be provided in parallel with a local load element to reduce the extent to which time varying voltage droop and ground bounce produce ripple in the voltage level applied to one or more nearby loads. Decoupling capacitors are useful where possible, to reduce local voltage droop and ground bounce by supplying current or sinking current according to time constants determined by the power supply conductor resistances, the load resistance, and the capacitance of the decoupling capacitor. But each decoupling capacitor is another load between the power supply rails and adds to the total load on the power supply. It may not be convenient or possible to provide effective decoupling capacitors for local load elements within an integrated circuit package.
Integrated circuit designers advantageously pay attention to the layout of loads and the supply voltage (VCC and/or VDD) and ground (GND) busses. There is an interest in keeping devices and conductors small and closely packed. Various rules are applied in view of operational tolerances and nominal component specifications. These rules translate into rules and tolerances for the size and length of supply and ground conductors, including by direct application of Ohm's Law.
For example, a regulator supplying an integrated circuit may have given output voltage tolerances. The supply voltage(s) may be used to drive circuit elements that likewise have a nominal operating supply voltage range and nominal maximum/minimum load current specifications. These specifications can be used to determine parameters such as the maximum conductor length, using iterative application of Ohm's law to determine the voltage drops that are encountered, so that the cascading effects of tolerances and voltage drop do not allow the operational supply voltage to droop below some predetermined level considered operationally adequate.
Voltage drop (V=IR) calculations may be complicated in the design of high current switched devices, densely populated circuits, and other applications. In such circuits, dimensional tolerances are tight and there is substantial incentive to make conductors narrow and thin. Circuit elements that are coupled to the power rails at points close to other elements tend to affect the local supply voltage droop and ground bounce conditions of the other elements nearby.
Certain CAD tools are available to aid integrated circuit designers. The Voltage Storm products of Cadence Design Systems Inc., for example, are intended to provide IR drop analysis as a part of the analysis of local power levels under dynamically varying conditions. The Voltage Storm product is configured with power calculation algorithms to enable calculation of the effects of loading, leakage, internal and switching power consumption and the like. The Voltage Storm product includes a “PowerMeter” function that is directed to the problems associated with the tight dimensions and high switching speed operation of leading-edge process technologies, which have little tolerance for IR drop.
Such CAD tools are complex and expensive, partly because the analysis is dynamic and affected by the switching operation of the circuit in question. The output is a proposed design dictated by tolerances, i.e., a design or layout wherein the voltage droop and ground bounce are expected to keep all the circuit elements in compliance with their maximum and minimum supply voltage tolerances. It would be advantageous to provide techniques in addition to computational analyzers that dictate tolerances so as to maintain a nominal state, for dealing with IR drop problems.
Apart from designing to meet tolerances, voltage droop and ground bounce considerations can be considered to affect the best performance that can be obtained from a circuit even when meeting tolerances. An example of a high speed digital integrated circuit that is designed to operate at the limits of speed and functionality, is the Agere Vision X100 digital processor for cellular applications. This processor is embodied substantially in a very large scale integrated circuit, and has a supply voltage sensitivity versus operational speed estimated at 4 mV/MHz. That is, for every 4 mV of droop on the supply potential and/or bounce on the ground potential, the dependable operational clock speed of the device is reduced by about 1 MHz. The processor draws 300 to 400 mA supply current during call processing. The total current loading can ramp up from 5 mA in a quiescent state to the full call processing loading level in just several nanoseconds. Assuming that there is as little as 10 milliohms of resistance between the VDD regulator output, the device, and GND, then 1 MHz of possible speed and performance is lost due to supply voltage droop and ground bounce.
It would be advantageous to provide some relief from the compounded demands or circuit speed, design complexity, power consumption that are associated with IR voltage droop and ground bounce.